ABSTRACT
The ras proteins are members of the family of guanine nucleotide-binding proteins (GTP-binding proteins); they exhibit guanine nucleotide-binding activity and possess an intrinsic GTPase activity. 1 - 4 As in the case of heterotrimeric G proteins, these biochemical properties of ras proteins are central to their biological function: the GTP-bound form is an active conformation that can activate a target protein, a stimulation shut-off upon hydrolysis of GTP to GDP. The GTP-bound form is regenerated through the exchange of bound GDP for external GTP, a reaction probably promoted by an upstream signal (Figure 1). Thus, by analogy to G proteins, ras proteins are thought to function as signal transducing molecules involved in cell growth control. To understand the detailed mechanism of the ras-mediated signal transduction pathway, it is important to identify the upstream and downstream components interacting directly with ras; none, as yet, have been identified in mammalian systems.
